In 2011, the ITU-T officially began a project to define advanced high speed transmission on twisted pair cables to address high speed transmission on short loop lengths (<250 m) at speeds up to approximately 1 Gb/s aggregate (sum of upstream and downstream rates). The result of this study is ITU-T Recommendation G.9701 (i.e. G.fast), which has since been adopted as a standard, and defines a transceiver specification based on time division duplexing (TDD) for the transmission of the downstream and upstream signals in a wide bandwidth of approximately 106 MHz and a symbol rate of approximately 48 kHz. This contrasts with prior standards such as VDSL2 having a 17.6 MHz bandwidth with a corresponding symbol rates of approximately 4 kHz and 30 MHz bandwidth with a corresponding symbol rate of 8 kHz.
In an effort to obtain power savings in a distribution point unit (DPU) with an option to operate with reverse power feed from the customer premises equipment (CPE), G.fast defines a scheme called discontinuous operation (DO). This allows transceivers on each link to “turn off” system processing to help scale the system power dissipation commensurate with the amount of data traffic being passed. By transmitting data in time slots when data is available and transmitting silence when there is no data available, the equipment power dissipation may be scaled directly with the available user payload data.
Although the power savings of DO is beneficial, it comes at the cost of requiring the vectoring system to maintain a matrix of Far End Crosstalk (FEXT) coefficients for all operating lines in the vectoring group, as well as a separate and distinct matrix of FEXT coefficients for only the lines in the DO group.
Moreover, vectoring systems such as G.fast need to perform a full estimation of the Regular Operation (RO) coefficient matrix every time any line joins or leaves the vectoring group. For example, with N1 lines in the G.fast vectoring system, an (N1×N1) FEXT coefficient matrix is engaged in the vectoring system to achieve full cancellation. As a few more lines join the system, a (N2×N2) coefficient matrix (N2>N1) would need to computed and engaged for full cancellation during RO. This transition from an (N1×N1) to an (N2×N2) matrix is preferably seamless, from one symbol to the next. This atomic switch (on a per-tone basis) is necessary to avoid vectoring-related, joining transients due to second order FEXT coupling. This imposes an additional coefficient memory requirement on the system, in particular for a N2×N2 “staging area”, to enable an atomic switch of coefficients from one matrix to another. This staging area is conventionally provisioned in addition to the separate and distinct matrix of FEXT coefficients for lines in the DO group, as well as the existing N1×N1 matrix for the current RO group.
What is needed, therefore, is a scheme that manages this complexity without additional memory requirements.